There are dozens of sizes of envelopes available. Not all are used for posting mail, but for such things as pay packets or putting a gift card in.
Paper envelopes were developed in China, where paper was invented by 2nd century BC. Paper envelopes, known as chih poh, were used to store gifts of money.
Up until 1840, all envelopes were handmade, each being individually cut to the appropriate shape out of an individual rectangular sheet. Most of the over 400 billion envelopes of all sizes made worldwide today are machine-made.
The A Series standards apply to envelopes as well as paper.
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